Color image receiving tubes with clear and high contrast picture images have been requested. However, the reflectance of the phosphor screens of image receiving tubes is almost 100%, and a non-luminescent section of the screen becomes white instead of black when outside light such as sunlight or incandescent light hits the screen, thus reducing picture image contrast. Thus, in order to improve picture image contrast, a face-plate with low light transmittance is applied to a color image receiving tube, or a colored film is coated on the outside surface of a face-plate. Contrast is improved if outside light reflected by a phosphor screen is absorbed by the face-plate or the colored film.
Also, in Laid-open (Kokai) Japanese Patent Application No. Hei 6-275209, the reflective brightness of a phosphor screen is reduced by coating a small amount of a pigment on the phosphor screen while maintaining the emission brightness (luminance) of the screen as much as possible. Examples of reducing the reflective brightness of a phosphor screen are disclosed in Kokoku Japanese Patent Applications No. Sho 52-14587, No. Sho 52-14588 and No. Sho 52-14589. In one example, pigment layers are placed between a face-plate and three color phosphor screens so as to match the color of each pigment layer to the color of each phosphor screen. In this case, incident light components having different colors from the colors of the phosphor screens are absorbed by the pigment layers, and the absorption of phosphor screen emission is also minimized. Thus, by setting the light transmittance of the face-plate high, contrast is enhanced without reducing the emission brightness of picture images.
The above-described conventional methods, however, have the following problems.
(1) In the method of applying a face-plate with low light transmittance or coating a colored screen on a face-plate, outside light reflected by a phosphor screen is absorbed by the face-plate or the colored screen, thus providing black picture images. However, even at a luminescent section of the phosphor screen, emission from the screen is absorbed by the face-plate or the colored screen, thereby reducing the emission brightness of picture images.
(2) In the method of coating a small amount of a pigment on a phosphor screen while maintaining the emission brightness of the phosphor screen, the reflective brightness of the pigment coated phosphor is about 75% of that of the phosphor with no pigment coating, and this is high. Thus, in order to increase contrast, a face-plate with low light transmittance or a colored screen has to be also applied. As a result, the emission brightness of picture images declines.
(3) In the method of locating pigment layers between a face-plate and three color phosphor screens, the processes of manufacturing the pigment layers are so complex that the cost of producing color image receiving tubes increases.